This conceptual model shows how porphyrin COFs embedded in a cathode could be used to split carbon dioxide into carbon monoxide and oxygen for making renewable fuels and other valuable chemical products. Image: Omar Yaghi, Berkeley Lab/UC Berkeley.
This conceptual model shows how porphyrin COFs embedded in a cathode could be used to split carbon dioxide into carbon monoxide and oxygen for making renewable fuels and other valuable chemical products. Image: Omar Yaghi, Berkeley Lab/UC Berkeley.

Researchers at the US Department of Energy (DOE)'s Lawrence Berkeley National Laboratory (Berkeley Lab) have created a molecular system that not only absorbs carbon dioxide but also selectively reduces it to carbon monoxide. The advantage of doing this is that, unlike carbon dioxide, carbon monoxide can be used as a building block in the production of a wide range of chemical products, including fuels, pharmaceuticals and plastics.

To produce their system, the researchers incorporated molecules of carbon dioxide reduction catalysts into sponge-like crystals known as covalent organic frameworks (COFs). "There have been many attempts to develop homogeneous or heterogeneous catalysts for carbon dioxide, but the beauty of using COFs is that we can mix-and-match the best of both worlds, meaning we have molecular control by choice of catalysts plus the robust crystalline nature of the COF," explains Christopher Chang, a chemist with Berkeley Lab's Chemical Sciences Division and a co-leader of this study.

"To date, such porous materials have mainly been used for carbon capture and separation, but in showing they can also be used for carbon dioxide catalysis, our results open up a huge range of potential applications in catalysis and energy," he adds. Chang and Omar Yaghi, a chemist with Berkeley Lab's Materials Sciences Division who invented COFs, are the corresponding authors of a paper in Science that describes this research in detail.

The notoriety of carbon dioxide for its impact on the atmosphere and global climate change has overshadowed its value as an abundant, renewable, non-toxic and non-flammable source of carbon for the manufacture of widely-used chemical products. But because carbon dioxide does not readily take part in chemical reactions, it first needs to be reduced to the more reactive carbon monoxide.

With this in mind, in 2005 Yaghi and his research group at the University of Michigan designed and developed the first COFs as a means of separating carbon dioxide from flue gases. A COF is a porous three-dimensional crystal consisting of a tightly folded, compact framework that features an extraordinarily large internal surface area – if a COF the size of a sugar cube were unfolded, it would cover a football field. The sponge-like quality of a COF's vast internal surface area allows the system to absorb and store enormous quantities of targeted molecules, such as carbon dioxide.

In this latest work, the researchers also took advantage of another technique developed by Yaghi called ‘reticular chemistry’, which provides a way for molecular systems to be ‘stitched’ into net-like structures that are held together by strong chemical bonds. This allowed them to embed a porphyrin catalyst, a ring-shaped organic molecule with a cobalt atom at its core, into the molecular backbone of COFs. Porphyrins are electrical conductors that are especially proficient at transporting electrons to carbon dioxide.

"A key feature of COFs is the ability to modify chemically active sites at will with molecular-level control by tuning the building blocks constituting a COF's framework," Yaghi says. "This affords a significant advantage over other solid-state catalysts where tuning the catalytic properties with that level of rational design remains a major challenge. Because the porphyrin COFs are stable in water, they can operate in aqueous electrolyte with high selectivity over competing water reduction reactions, an essential requirement for working with flue gas emissions."

In performance tests, the porphyrin COFs displayed exceptionally high catalytic activity – a turnover number up to 290,000, meaning one porphyrin COF can reduce 290,000 molecules of carbon dioxide to carbon monoxide every second. This represents a 60-fold increase over the catalytic activity of a molecular cobalt porphyrin catalyst and places porphyrin COFs among the fastest and most efficient catalysts for reducing carbon dioxide. Furthermore, the research team believes there's plenty of room for further improving the catalytic abilities of the porphyrin COF.

"We're now seeking to increase the number of electroactive cobalt centers and achieve lower over-potentials while maintaining high activity and selectivity for carbon dioxide reduction over proton reduction," Chang says. "In addition we are working towards expanding the types of value-added carbon products that can be made using COFs and related frameworks."

This story is adapted from material from the Lawrence Berkeley National Laboratory, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.